The field of the invention is disc memory devices generally. Specifically, the invention relates to disc memories having detachable disc modules.
In the prior art, the so called Date Module produced by International Business Machines Corporation is the most similar device now existing. It comprises a cantilevered spindle carrying the recording discs and a sliding carriage carrying the head arms, all contained within a plastic housing. When mounted on the appropriate drive unit a belt drives a shaft interlocking with the spindle. An actuator mechanism engages the head arm carriage to position the head arms for accessing the various tracks on the disc. The spindle is cantilevered on two bearings interposed between the disc and the drive shaft attachment point.
A servo read head on one surface of a servo disk containing prerecorded servo tracks provides a head-to-track error signal to the servo control loop. The servo control loop located in the drive unit provides a signal to the actuator to accurately locate the servo head over the desired servo track. The data tracks on the remaining disk surfaces are written and read when the servo head is properly located over the desired servo track. Mechanical stresses and thermal growths can cause positional errors between the data heads and their respective tracks even though the servo head-to-track error is zero.
U.S. Pat. Nos. 232,735D; 3,710,357; 3,786,454; 3,839,920; and 3,843,967 disclose pertinent aspects of said Data Module. In the Data Module, the belt drive on the spindle as shown in U.S. Pat. No. 3,839,920 imposes lateral loads affecting the predetermined accuracy of the spindle position. Furthermore, in the mechanical attachment of the actuator to the head arm carriage, the hardened steel magnetic parts which are required to achieve acceptable wear characteristics cause a varying magnetic bias force on the carriage due to actuator magnet structure leakage which in turn produces proportional servo errors as the carriage moves backward and forward.
A rotary motion head arm assembly with a mechanically connected actuator also requires a complex connecting mechanism. The connecting mechanism will decrease servo loop performance and introduce adverse effects which will result in positional errors. When dealing with track densities as high as 1,000 tracks per radial inch, even small deviations in head to disk alignment across the spindle length can drastically affect the overall positioning accuracy.